The bacteriophage sensitivity of the Streptococcus bovis population resident in the rumens of two fistulated sheep was monitored for 112 days. During this time, three changes in the bacteriophage sensitivity of S. bovis occurred in the absence of detectable bacteriophages. Identical changes in bacteriophage sensitivity occurred simultaneously in both animals and, except for the relatively short periods of changeover in phage sensitivity, the S. bovis population in the rumens of the two sheep was homogeneous with respect to phage sensitivity.